This invention generally relates to interferometer spectrometers and more specifically to improved techniques for determining the presence and magnitude of particular discrete frequency components of light of unknown spectral content, and to an improved absorption gas cell and source for use therewith. In a particular application, this invention finds use for the rapid determination of the composition of engine exhaust gasses by means of an interferometer spectrometer system which measures the amount of light absorbed by exhaust gasses at discrete, preselected frequencies in the infrared. As used herein, light includes electromagnetic energy throughout the infrared, visible and ultraviolet, although the specific system disclosed is directed to use in the infrared.
Heretofore, the analyses of spectra containing information concerning the emission or absorption of light have been used to identify the material generating the light or a material through which the light has been passed and been partially absorbed. As an example, infrared spectral analysis of absorption finds particular use in gas sample analysis, such as could be employed in in the studies of exhaust from factories or engines. In the latter application there is a large potential use for accurate analysis of automotive exhaust. In such applications certain discrete frequencies of emission or absorption can be correlated to the unknown quantity of a constituent of the gas being analyzed. In automotive exhaust analysis, certain constituents of the exhaust gas are responsible for its smog producing potential and can be analyzed by passing a wide band light beam through the exhaust gas and measuring the light attenuation at particular absorption frequencies which characterize these constituents.
Interference spectroscopy has been useful for measuring infrared light and has been known for some time. In general, the interference fringe pattern of the interferometer is one of a Fourier transform pair with respect to the frequency spectra of interest, and could be transformed in a general purpose computer to yield the spectra. This is unduly expensive.
U.S. Pat. No. 3,482,919 to Anthony R. Barringer suggests passing light through a gas sample and into an interferometer, and then using correlation techniques for comparing time varying signals derived from the light emerging from the interferometer with correlation signals characteristic of light of known spectral content.
While Barringer's method can be used to measure the presence and amount of a particular gas, several measurements would have to be performed on each engine exhaust sample, one for each gas whose presence in the sample is suspected. It is desirable to have a system which can determine the amounts of numerous suspect gasses with one test of each sample. This would reduce the amount of time involved as well as the cost particularly in the automotive industry.
U.S. Pat. No. 3,286,582 to Mertz describes a system which provides frequency spectrum information regarding the light entering the interferometer by moving a reciprocating mirror at a uniform velocity. He thereby generates a time function comprised of audio frequency components, the amplitude of which are representative of the energy of corresponding wavelength components of the incoming light. The frequency of the audio components corresponding to a component of this incoming light is given by the ratio of the uniform mirror velocity to the wavelength of that corresponding light component.
However, the information provided covers the complete frequency spectrum and is more than that necessary to determine the amount of selected components. Furthermore, the dependence on uniform movement of the moving mirror renders the system unduly delicate and, therefore, insuffiently accurate.
There is, therefore, a need for an accurate interferometer adapted for measuring the frequencies of absorption or emission contained in a spectra of a material to thereby generate reliable data on the amount of preselected components contained in the material.
In testing for the impurity content of engine exhausts by interferometric means, it is desirable to pass the sample through a light absorption chamber or cell such that incoherent light is energized to follow a multiple or long path through the cell and the components or light having frequencies characteristic of the gaseous components present will be absorbed by the impurities to a certain extent, depending on the impurity content in the sample. In one such cell, opposing mirrors provide a folded optical path which allows many traverses of the light across the path of the gas. One disadvantage of the direct use of this cell is the mirrors can become readily covered with grime in use, particularly with engine exhaust samples and reduce the accuracy and efficiency of the system.
Solid particles contained in exhaust and whose diameters are on the order of the wavelengths corresponding to the absorption frequencies of the impurities being measured are present in samples and interact with light at those frequencies thereby adversely affecting the measurements. Such particles could also accumulate on the mirrors of the absorption cell. Therefore, it is desirable to remove such particles before passing the sample through the absorption chamber and to control the flow through cell to reduce deposition of grime on the mirrors. Since the particles contribute to air pollution, it is desirable that these particles should be collected and counted or weighed to determine their contribution to the impurity content of the sample. The particular impurities typically found in engine exhaust samples have absorption frequencies in the infrared portion of the electromagnetic energy spectrum, therefore, the light source should emit light which includes all of these frequencies. Conventional light sources either have an enclosure window such as glass or quartz which blocks some of the desired light, or an exposed source which oxidizes and burns out quickly. There is a need therefore, for an enclosed source which provides a broad band light output and which has a long lifetime.